A WDM (Wavelength Division Multiplexing) transmission system is in practical use as one form of an optical transmission system. With WDM, a plurality of optical signals having different wavelengths are multiplexed and transmitted. As a method or a configuration for recovery from a transmission line failure in a WDM transmission system, an OUPSR (Optical Unidirectional Path Switched Ring) is known.
FIG. 1 is an explanatory view of the OUPSR. In the example illustrated in FIG. 1, a WDM transmission system includes node equipments A to I, which are connected in the form of a ring. Moreover, the node equipments A to I can respectively transmit an optical signal in a clockwise direction and a counterclockwise direction.
The following description assumes that data is transmitted from a client X to a client Y. In this case, the node equipment A guides an optical signal X1 to the node equipment B, and also guides an optical signal X2 to the node equipment I. Namely, the optical signal X1 is transmitted in the counterclockwise direction, whereas the optical signal X2 is transmitted in the clockwise direction. The optical signals X1 and X2 are substantially the same each other, and propagate data transmitted from the client X. Moreover, each of the optical signals X1 and X2 is multiplexed into a corresponding WDM optical signal and transmitted.
The node equipment F extracts the optical signal X1 from the WDM optical signal received via the node equipments B, C, D and E, and guides the extracted optical signal X1 to an OUPSR device. Moreover, the node equipment F extracts the optical signal X2 from the WDM optical signal received via the node equipments I, H and G, and guides the extracted optical signal X2 to the OUPSR device. Namely, the OUPSR device receives both of the optical signals X1 and X2. Then, the OUPSR device selects one of the optical signals X1 and X2, and guides the selected signal to the client Y. Here, assume that the OUPSR device selects the optical signal X1 and guides the selected signal to the client Y.
Assume that the optical signal X1 transmitted from the node equipment A in the counterclockwise direction does not reach the node equipment F due to a failure in the WDM transmission system with the above described configuration. Here, the OUPSR device continuously monitors reception levels of the optical signals X1 and X2. If the reception level of the optical signal X1 becomes lower than a specified threshold, the OUPSR device detects LOL (Loss of Light) of the optical signal X1. Then, the OUPSR device guides the optical signal X2 received via the node equipments I, H and G to the client Y. Namely, the OUPSR device switches an optical path for transmitting data from the client X to the client Y. In this way, the OUPSR device switches an optical path when a failure has occurred on a transmission line, whereby the data transmission is recovered.
Note that optical path switching performed due to a failure can be implemented with another method. For example, node equipment that has detected LOL of a certain wavelength channel may notify a failure signal to an OUPSR device that drops the wavelength channel. In this case, the OUPSR device switches an optical path of the wavelength channel according to the failure signal.
Related techniques are as follows. International Publication Pamphlet No. WO2009/022406 describes an optical receiver that can instantaneously detect optical signal LOL of an optical channel. International Publication Pamphlet No. WO2005/008924 describes a transmission route switching control method. Japanese Laid-Open Patent Publication No. 2009-130505 describes a method of setting a threshold for detecting an optical input LOL. Japanese Laid-Open Patent Publication No. 2008-252657 describes a method for identifying a failure in a connection between domains in an optical network.
As described above, optical path switching is implemented, for example, by monitoring an optical level of a received signal in an OUPSR device. However, for example, when a transmission line from a location where LOL of a certain wavelength channel has occurred to a node (namely, a drop node) that drops the wavelength channel from a WDM optical signal is long and many nodes are provided on the transmission line, the OUPSR device cannot suitably monitor a level of an optical signal in some cases.
Assume that a failure occurs in the optical signal X1 in the node equipment A in the example illustrated in FIG. 1. Since the optical signal X1 is dropped from the WDM optical signal in the node equipment F and guided to the client Y, the node equipment F is a drop node of the optical signal X1. In this case, the node equipment F drops a wavelength (hereinafter referred to as λx allocated to the optical signal X1, and guides the signal to the OUPSR device.
Incidentally, each of the node equipments includes an optical amplifier to amplify a WDM optical signal. The optical amplifier generates ASE (Amplified Spontaneous Emission) noise. Accordingly, ASE Noise is accumulated in the node equipments B, C, D and E arranged on a transmission line from the node equipment A to the node equipment F. Therefore, when the node equipment F drops the wavelength λx from the WDM optical signal and guides the signal to the OUPSR device in a case where the optical path of the optical signal X1 is disconnected, an optical level detected by the OUPSR device sometimes becomes higher than a threshold for detecting LOL. In this case, the OUPSR device cannot detect LOL of the optical signal X1. Therefore, the optical path is not switched. Namely, a data communication between the clients X and Y is not recovered.
In the meantime, to switch an optical path by using the above described failure signal, not only the node equipments but the OUPSR device needs to support recovery protocol using the failure signal. However, it is preferable that each of the node equipments can accommodate an arbitrary communication device in order to flexibly configure an optical transmission system. Accordingly, an OUPSR device that does not support the recovery protocol using the failure signal is sometimes connected to a node equipment. Since the OUPSR device cannot detect a failure in this case, an optical path is not switched.
The above described problem does not occur only in an optical transmission system employing an OUPSR. That is, the above described problem may occur in an optical transmission system where an optical path is switched due to a failure.